Disk system and power-on sequence for the same

ABSTRACT

With respect to the disk drives provided power from a single power source in a disk drive system, start up power is first supplied to a first start-up group of the disks, preferably comprising all of the master disks, with the size of the group being selected so that the required current does not exceed the capacity of the power source. When the disk drives of the first group have substantially reached steady state, start-up is conducted with respect to a second start-up group of the disk drives so that the current required during start-up for the second group and the current required for steady state drive of the first start-up group does not exceed the capacitv of the power source. With respect to each start-up group, the number of disk drives is the maximum integer value and decreases or remains the same with respect to subsequent start-up groups. When simultaneously transferring subdivided data in parallel to all of the disk drives of a parity group, respectively, seek operations in such a system are prevented from occurring simultaneously by offsetting the indices on the disks, by varying the seek operation start timing or by varying the head addresses for the start of reading and writing, all within one revolution, but the seek operations are ended at the same time, to reduce peak current requirements of the power source.

BACKGROUND OF THE INVENTION

[0001] This invention relates to an external memory unit for a computeror high-per or,.mance computer system, and more particularly to an arraydisk system employing a large number of small disk drives and a maximumpower supply current requirement control, e.g. with respect to power onor head seek.

[0002] In current computer systems, the data required by the host side,e.g., by the CPU (central processing unit), is stored in a secondarystorage system and the data is written to and read from the secondarystorage system as required by the CPU.

[0003] The increasing sophistication of information systems in recentyears has led to a need for higher performance secordary storagesystems. One answer to this need is the array disk system which, as willbe clear from the following descripton, consists of a large number Carelatively small capacity magnetic disk drives. The array disk system isused for conducting parallel processing. Specifically, the datatransferred from the CPU is subdivided and sinultaneouslv stared in aplurality of magnetic disk drives and, during data read, the subdivideddata is simultaneously read from the magnetic disk drives regeneraed toobtain the original data from the data read simultaneously from the diskdrives and transferred to the CPU at high speed. The magnetic diskdrives that carry out this parallel processing are divided into groupsas indicated in FIG. 12(a). Each group constitutes a unit within whichall member magnetic disk drivers operate in the same manner.

[0004] The secondary storage system generally uses nonvolatile storagemedia, typically magnetic disk drives, optical disk drives or the like.

[0005] This type of array disk system is discussed, for example, by D.Patterson, G. Gibson and R.H. Kartz in a paper titled A Case forRedundant Arravs of Inexpensive Disks (RAID) read at the ACM SIGMODConference, Chicago, Ill., (June 1988). This paper reports on theresults of studies into the performance and reliability of both arraydisk systems which subdivide and process data parallely and array disksystems which indeoendently treat distributed data. The two array disksystems referred to in this paper are considered to be the most commontypes in use today.

[0006] The array disk system which subdivides data and processes thesubdivided data parallely will now be explained. The array disk systemhas a large number of relative small capacity magnetic disk drives. Asshown in FIG. 14, the data transferred from the CPU is subdivided andsimultaneously stored in parallel in a plurality of data disk drives 7and a parity disk drive 8 that constitute a parity group 4. During dataread, the procedure is reversed, i.e., the subdivided data issimultaneously read in parallel from the disk drives regenerated toobtain the original data from the data read simultaneously from the diskdrivesand transferred to the CPU. This parallel processing enables thedata to be transferred at high speed. For enhancing the reliability ofthe array disk system, parity data is generated from the subdivided dataand stored in the parity disk drive P (8). In this way, when a problemarises making it impossible to read data from one of the magnetic datadisk drives D (7) among those in which the subdivided data is stored,the data stored in the disabled magnetic disk drive can be reconstructedfrom the data stored in the remaining magnetic disk drives 7 and theparity data of disk drive 8. The provision cf parity disks is necessaryfor improving the reliability of a system which, like the array disksystem, consists of a large number of magnetic disk drives.

[0007] Systems in which a high transfer rate is realized bvsimultaneously conducting reading and writing with reszec- o anv arrayof disks are disclosed in Japanese Unexamined Patent ?Publ, Disclosure1(1989)-250158 and Electronic Design, Nov. 12, 1987, p. 45. As shown inFIG. 2, these types of systems define a plurality of disk drives 211-215as an array. Preferably a rotation synchronize circuit 220rotation-synchronizes these disk drives with respect to an externalreference clock or with respect to one disk drive among the plurality ofdisks making up the array. A sequencer 240 subdivides the datatransferred from the host 210 through an interface 230 into bits, bytes,blocks or some other arbitrary unit, and also generates parity or othersuch EC, (error checking and correction) data. These data are written tothe disk drives 211-214 substantially simultaneously by disk drivecontrol circuits 250. During regeneration, the sequencer 240reconstructs the original data from data read simultaneously from thedisk drives and outputs the regenerated data to the host through theinterface 230. The b 260 is situated between the control circuits 250and the sequencer 240 for absorbing rotational discrepancies among thedisks. The interface 230, sequencer 240, control circuits 250 andbuffers 260 are controlled bv a processor 270.

[0008] When reading and writing of data are conducted with respect toN=1 disks (+1 indicating the parity disk 215) in this manner, theapparent transfer rate becomes N times the transfer rate of theindividual disk drives. Moreover, the provision of a redundant disk (theparity disk 215 in this example) makes it possible to ensure accuratedata regeneration even if one disk adrive should break down.

[0009] Further, as shown in FIG. 3, COMPCON 189 Spring, Febuarary 1989,p118 discloses an arrangement in which a plurality of interconnecteddisk drive arrays 281-284 (which will be referred to as parity groups)are each constituted in the manner of FIG. 2. High-speed transfer isrealized by having the disks within the parity groups 281-284simultaneously conduct read and write operations. When a disk within agroup breaks down, the data is reconstructed within the group concerned.This reference further discloses the formation of separate groups291-295 (which will be referred to as power groups) constitutedperpendicular to the parity groups. Each power group constitutes aseparate unit as regards the supply of electric power for toe diskdrives and the cooling fans. This arrangement limits the effect of thebreakdown of a single power group to making it impossible to read thedata of only one disk in each parity group. As a result, the aforesaiddata error checking and restioration capabilitv remains intact and thedata can be regenerated.

SUMMARY

[0010] The aforesaid arrangements do not, however, take into account thefact that the initial current becomes large when the large number ofdisk drives are simultaneously started up. As shown in FIG. 4, the powersupply current required immediately after start-up of a disk drive ismore than twice that during steady state operation. This large currentfollowing start-un continues to flow for no more than several tens ofseconds. Assume that a single power supply serves D number of diskdrives (D being eaual to the number of parity groups), that the steadystate current value is I(A) , and that a current equal to k times thesteady state current is required immediately after start-up. The powersupply is thus required to be capable of supplying, albeit for only ashort period, a current of I x k x D (A).

[0011] Japanese Unexamined Patent Publication Disclosure 57(982)3265discloses a technique for staggering the times at which cower-on isconducted with respect to the disk drives. While this method makes itpossible to reduce the required capacity of the power supply, itconsiderably prolongs the time reauired for start-up of the entiresvstem when applied to a svstem which, like the array disk system, has alarge number of disk drives that have to be supplied with power.

[0012] An object of this invention is to provide an array disk systemand control the same to reduce the amount of electric current requiredby the array disk system, e.g. the amount of electric current requiredthereby during a power-on sequence for the disk system which enables thedisk system to be started up within a prescribed period of time usingrelatively small power supplies.

[0013] For achieving this object, the present invention divides the diskdrives within the disk system into a number of groups and separatelystarts up the respective disk drive groups.

[0014] The number of disk drives constituting the individual groupsordinarily decreases in the order that the groups are started up. Thisis because, for example, the reserve power c: the power supply after thestart-up of the first group is equal to the rated capacity of the powersupply minus the amount of current required for maintaining the diskdrives of the first group in the steady state. It suffices to set thenumber of disk drives in the first grouo to be started up so as not toexceed the capacity of the power supply being used. This number can bedecided by the following method.

[0015] Assume that D disk drives are started up using a single powersupply, that the steady current per disk drive in the steady state isI(A) , and that an initial current k times as large as the steady statecurrent is reauired at the time of start-up. Then, if the number of diskdrives first started up is set at D/k, the current at the time ofstart-up will not exceed the amount of current when all of the diskdrives are oacerating in the steady state, namely, will not exceedID(A).

[0016] Next, the manner for determining the number of disks to beincluded in the second and following groups to be started up will beexplained. Basically, it suffices if the number of disk drives in thesecond and following groups to be started up is such that the amount ofcurrent required for starting up the disk drives does not exceed thereserve capacity of the power supply. For optimum effect, however, thefollowing method can be considered. After the first group of D/k diskdrives have reached the steadv state (e.g., after several tens ofseconds), the next group of disk drives is started up. It then basicallysuffices to set the number x of disk drives in this next or second groupas the number obtained by dividing the reserve current capacity of thepower supply when D/k disk drives are operating in steady state by kI.This can be expressed bv the following equation:

x=1/k (1-1/k)D

[0017] Since only an integral number of disk drives is possible, anydecimal amount in the value of D/k is dropped, i.e., the value obtainedfrom the foregoing equation is rounded down. When this method is usedfor determining the numbers of disk drives, it mav happen that a singledisk drive remains at the end. For starting up this disk drive, however,a maximum power supplv current of I(D−1+k) (A) is sufficient.

[0018] One disk drive of a parity group is sometimes designated as amaster disk and subjected to rotation synchronizatio4n. In such case,this master disk has to be started up prior to the other disks. If thenumber of master disk drives is such that they can all be started upsimultaneously, therefore, the master disk drives are included in thefirst group to be started up. Alternatively, it is possible to start upthe master disk drives one by one before starting up the other disks.

[0019] Since the disk drive groups are started up at different times toprevent overlap of the initial currents, the maximum current output ofthe power supply can be reduced. Since the disk drives are organizedinto a number of groups, the disk system can be started up within aprescribed period of time.

[0020] An example magnetic disk drive of a type illustrated hereinrequires a maximum current of 4.5A, which breaks down to ! A forrotating the disks, 2.8 A for seek operation and 0.7 A for otherpurposes. When seek operation occurs simultaneously with parallelprocessing in an array disk svstem consisting of a large number of suchdisk drives, a very large current becomes necessary. Moreover, asprotection against power outages or other such mishaps that might occurduring the operation of such an array disk system, it is necessary toprovide battery backup for enabling data in the course of storage to becompletely stored. For supply of such a large amount of current, it isnecessary to use a verv large battery.

[0021] An object of this invention is to provide an array disk systemand control the same to reduce the amount of electric current requiredby the array disk system, particularly the amount of electric currentrequired thereby during seek operation, and also in this way to reducethe capacity required of a battery provided as a backup power source foruse during power outages and the like.

[0022] For achieving the aforesaid object, the present inventionprovides an array disk system, as shown, for example, in F gures 12(a),(b), and (c) that has a large number of disk drives divided into aplurality of groups provided with contsrl such that the timing of thestart of seek onerations for movina the read/write heads to change thetrack positions at which the read/write heads are located is variedamong at least some of the groups and such that, within each group, thetiming of the star of seek operations is the same for all of the diskdrives or is varied among at least some of the disk drives.

[0023] The control for causing the seek operation start timing to varyamong the groups or among the disk drives of a group can be provided byrotation-synchronizing the disk drives such that the positions ofindices provided on the disks as references for the start of dataread/write are offset among the groups or among the disk drives.

[0024] In this case, parallel processing can be readily conducted bvproviding the controller with data processing which simultaneouslystores the subdivided data simultaneously transferred to the respectivegroups in buffer memories within the respective groups and conductsread/write processing of the data from the buffers in accordance withthe positional offset of the indices.

[0025] Alternatively, the control for causing the timing of the start ofthe seek operations to vary among the groups or among the disk drives ofa group can be provided, as shown in FIGS. 18(a) and (b) for example, bydeliberately offsetting the seek operation start timing among the groupsor among the disk drives, without offsetting the positions of theindices on the disks. Since all of the indices are positionally alLgnedin this case, there is the advantage that rotation synchronizationcontrol is easy to conduct.

[0026] Further, the control for caus ng the timing of the start oL theseek operations to differ among the groups or among the disk drives of agroup can be provided, as shown in FIGS. 19(a) and (b) for example, byvarying the head addresses for the start of data reading and writingamong the groups or among the disk drives, without offsetting thepositions of the indices on the disks among the groups. This method smplifies the control since the head addresses can easily be varied amongthe groups by software techniques.

[0027] The control used by the invention for achlevina the aforesaidobjects is further characterized in that the seek operations for movingthe read/write heads to change the track positions at which the headsare positioned are prevented from occurring simultaneously in at leastsome of the disk drives.

[0028] For preventing seek operations from occurring simultaneously thecontrol will offset the position of the indices on the dhsass to varythe seek operation start timing or vary the head addresses for the startof reading and writing.

[0029] In preventing seek operations from occurring simultanecusly, itis preferable from the point of reducing electric power consumption todivide the large number of disk drives into group units, each of aplurality of the disk drives, to prevent seek operation from occurringsimultaneously among the groups, and to make the division of the diskdrives into groups such that the seek operations occur in differentgroups at different times within the period of one disk revolution andall of the seek operations occurring at different times are completedwithin the same period.

[0030] In a disk system which conducts parallel processing, thepositional relationship among the heads situated over the disks isgenerally such that the many disk drives making up the svstem operate asif they were an integrated unit. Specifically, the disks arerotation-svnchronized with each other and the heads operate such thattheir track position relationships are all the same. In such a system,if the many disk drives which conduct parallel processing are dividedinto a number of groups and each group is treated as a separateread/write unit, the time for conducting seek operation is offset amongthe groups so that the occurrence of a large seek current bv thesimultaneous occurrence of the many seek currents in the individualdisks can be avoided. Therefore, the supply of current to the array disksystem as a whole is lowered and the capacity required of a battery forproviding backup power during power outages and the like can be reduced.

[0031] Offsetting the positions of the indices on the disks among thegroups makes it possible to offset among the groups the timing at whichseek operation starts for data exchange between the heads and the tracksduring one revolution and, thereby, to hold the seek current to a lowlevel.

[0032] As explained above, a prescibed seek time is required within eachrevolution for conducing a seek operation. During this time, the diskcontinues to rotate irrespective of whether or not data is beingexchanged. It is thus preferable o make effective use of this periodduring which data is not being processed for carrying out the seekoperation separately in each group. If this expedient is adopted, then,by deliberately offsetting the timing at which the seek operation isconducted among the groups within this period, it becomes possible,without offsetting the positions of the iudces₁ to use this period togood advantage and thus to reduce the seek current.

[0033] Since in one and the same disk drive the seek operatlon isconducted after the head at a specific head address (e.g., thebottommost head in FIG. 2) has completed data exchange with a track onthe disk, changing the head address at which data read/write is startedamong the groups changes the timing at which seek operation is conductedamong the groups, so that the seek current can be reduced.

[0034] Up to this point, the explanation has been directed to the casewhere the seek operation timing is varied among the groups. It is,however, similarly possible to reduce the seek current by varying theseek operation timing among disk drives in one and the same groupaccording to the above teachings.

[0035] Since reducing the seek current reduces the amount of electricpower that has to be supplied to the array disk system as a whole, itdecreases the capacity required of the backup battery for providingpower during power outages and the like, increases the reliability ofsystem operation during such emergencies, and enables the eauipment forsupplying power to be made more compact.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG 1. is a schematic view for explaining a disk svstem accordingto the invention;

[0037]FIGS. 2 and 3 are schematic views useful in explaining problemssolved by the present invention;

[0038]FIG. 4 is a graph for expla ning current characteristics of thespindle motor of a disk drive immediately after start-up;

[0039]FIG. 5 is a block diagram for explaining a motor drive controlcircuit;

[0040]FIG. 6 is a graph for explaining variation of the power supplycurrent;

[0041]FIG. 7 is a schematic view of a disk system for explaining theinvention;

[0042]FIG. 8 is a graph for explaining variation of the power supplycurrent in the invention;

[0043]FIGS. 9 and 10 are schematic views of systems for explaining theinvention;

[0044]FIG. 11 is a schematic view of another embodiment or theinvention;

[0045]FIG. 12(a) is a block diagram of a system portion of the inventionrelating to data storage;

[0046] FIGS. 12(b) and (c) are diagrams for explaining data storage inthe system of FIG. 12(a);

[0047]FIG. 13 is a schematic view of the interior of a data or paritydisk dive;

[0048]FIG. 14 is a diagram for explaining parallel processing of data;

[0049]FIG. 15 is a timing chart relating to data storage in a data orparity disk;

[0050]FIG. 16 is a block diagram showing the internal structure of agroup controller;

[0051] FIGS. 17(a) and 17(b) are diagrams for explaining another exampleof data storage in the present invention;

[0052] FIGS. 18(a) and 18(b) are diagrams for explaining another exampleof data storage in the present invention;

[0053] FIGS. 19(a) and 19(b) are diagrams for explaining another exampleof data storage in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0054]FIG. 13 shows the internal structure of a data disk drivepreferably used throughout this disclosure, for exampLe a data diskdrive 7 or a parity disk drive 8. A number of disks 12 rotate about acommon shaft 17, and R/W (read/write) heads 13 for reading and writingfrom and to the disks 12 are carried byan actuator 11. As used herein,the term head will refer to a single head for one surface as well as apair of heads that service opposed surfaces of adjacent disks. The heads13 are numbered, from top to bottom, #1 to i8. At least one R/W head 13is required per side for each disk 12. All of the R/W heads 13 are movedin unison by the actuator 11. When read or write is conducted, the CPU 1issues a data storage address and the R/W heads 13 go to this address.SpecifIcally, a head se7ector 14 selects the head number correspondIngto the head address included in the address issued by the CPU 1 and theactuator 11 carries out a seek operation by which the R/W head 13 ismoved to the track corresponding to the cylinder address. When access bvthe storage address for the data has been completed, a path selector 16selects the path to the host and the data is read or written by R/Wcircuit 15.

[0055] In a conventional manner, the disk drives 7, 8, 211-215, 411-414,440, 551-560, shown in FIG. 13, are referred to with a head address 30,and access is widn respect to a specific track 31 that defines acylinder for all o the disks 12. An index 32 is provided on one or moreor all of the disks 12 of the disk drive positioned to the head address30, and there is a cylinder address 33. Each single head or head pair issupported on an arm 34 having its other end supported on the actuator11, which moves radially with respect to the shaft 17 that rotatablysupports the disks 12. As shown by the headed arrows, data, address andcontrol signal lines are connected to a suitable bus, for exampleleading to the host.

[0056] In the case of an array disk svstem having the array diskcontroller ADC2 and array disk unit ADU3 of FIG. 12(a) which conductsparallel processing, the CPU 1 issues the read or write request to adisk drive parity group 4 made up of a number of data disk drives 7 anda parity disk drive 8 and which disk drive parity group 4 constitutes asingle parallel processing uni. Within the group 4, a read or writerequest is issued by the group controller GC5 to each data disk drive 7and the car4Ldisk drive 8, and read or write processing of the typedescribed above is conducted simultaneously with respect to all of thedata disk drives 7 and the parity disk drive 8 in the group 4. For thispurpose, it is necessary to rotation-synchronize the disks 12 of thedata disk drives 7 and partty disk drive 8 within the parity group 4 sothat the same address is always being accessed in the data disk drives 7and parity disk drive 8 within one access period, thereby to control themulti-disk system 2, 3 exactly as if it were a single disk drive.

[0057] An explanation will now be given on the problems that ariseduring reading and ritina of data in this type of array disk system whenthe volume of data to be handled at one time becomes great.

[0058] When data is stored in the data disk drives 7 and the parity diskdrive 3, it is first stored at the cylinder track under head #1 and thensuccessively stored at the same cylinder tracks under heads #2, 3, 4, 5,6, 7 and 8. When storage has been completed up to the cylinder trackunder head #3, the actuator 11 conducts a seek operation to move the R/Wheads 13 to the adjacent cylinder, wherein data is similarly storedsuccessively or in parallel at the tracks under heads #0, 1, 2, 3, 4, 5,6, 7, B. Reading of the stored data is carried ou in a similar manner.

[0059] Thus, in the array disk system, when the amount of data to besimultaneously processed in data disk drives 7 is larger than thecapacity of one cylinder or available area of the cylinder firstaccessed, an intermediate seek operation is necessary for moving to theadjacent track.

[0060] A system for the invention is shown in FIG. 1. The power supplycurrent (in lines 9 from power surply 10 of FIG. 12(a) e.g.) permagnetic disk drive immediatelvy after start-up in ths embodimentexhibits the characteristics shown in FIG. 4. The required power supplycurrent, for this example, is 2A during steady state operation and 4Aduring initIal state operation (start-up). The initial state current of4A continues to flow for 30 seconds. The manner in which the magneticdisk drives are arrayed in this example of FIG. 1 is similar to that inthe example illustrated in FIG. 3. each parity group 310 includes fivedisk drives, each steady state power group 320 includes eight diskdrives, and the total number D of disk drives is forty. As there arefive power groups, without the application of this invention it wouldordinarily be necessary to provide 5 power supplies, each with thecapacity to supply up to 4(A)×8 (drives)=32 (A). Being common to thewhole system, the interface 230 and the sequencer 240 should preferablybe provided with a power supply separate from that for the magnetic diskdrives so as to establish a dual power system. Even though the bufferand control cirouits are paired, with each pair connected with the diskdrives of a respective power group, thev are logical circuits and shouldtherefore have a different vo tace power supply. If to the contrary theyare to be suppliers with decreased voltage from the power supply forstarting up the disk drives, it is necessary to take the current theyrequire into account in determining the required current capacity of thepower supply. Herein, when we talk of current capacity or the like of apower supply we are really talking about that available for the diskdrives. Although the current required by these logical circuits varieswith their size, it is at any rate much smaller than the currentsrequired for driving the disk drive spindle motors. In this embodiment,the current required for the logical circuits is not more than 0.3 A.

[0061] When the present invention is applied, the number and seauence ofthe disk drives simultaneously started up in each power group 320 can be4, 2, 1, and 1. As shown in FIG. 1, the disk drives are organized fromthe top down into groups 330, 340, 350 and 360, consisting of 4, 2, 1and I parity groups 310 of disk drive(s), respectively. Further, as canbe seen in the motor drive control circuit shown in FIG. 5, the timebetween power switch-on of the overall system and the start of drivingof the disk spindle motors is set independently for each of the groups330, 340, 350 and 360 to prevent overlap of the initial currents amongthe groups. In the present embodiment, a delay of 30 seconds isestablished between successive grouos. The spindle motors 380 of group330 are turned on by a driver 390 almost simultaneously with receipt ofthe power-on signal 370. The spindle motors 380 of group 340 are turnedon after a t mer circuit 400 has counted off 30 seconds followingreceipt of the power-on signal 370. In the same manner, the spindlemcotrs 380 of group 350 are turned on after a delay of 60 seconds andthose of group 360 after a delay of 90 seconds.

[0062] The time course variation in the initial current of the powersupplies of the respective power groups during this process was as shownin FIG. 6. As can be seen, it rose no higher than 18A, which is roughly50% less than that should the invention not have been applied. The timerequired for all of the disk drives to reach their rated rotationalspeed was 30 seconds×4 groups=2 minutes. After start-up the steady statecurrent was 16A. In comparison, if an attempt should be made to limitthe power supply current to the same level without application of theInvention, the disk drives would have to be started up one by one andthe time required for all of the disk drives to reach their ratedrotational speed would be 30 seconds×8 disk drives=4 minutes. Thereduction in time is thus also 50%.

[0063] In the foregoing explanation the numbers of disks simultaneouslystarted up were 4, 2, 1 and 1. If the power supplies have adequatecapacity, however, it is alternatively possible to start up the diskdrives in three power groups of 4, 2 and 2 disks. While this increasesthe required amount of power supply current to 20 A, it reduces the timerequired for all disk drives to reach their rated rotational speed to 1minute 30 seconds. From this iw will be understood that the numbers ofdisks to be simultaneously started up can be varied in light of the sizeof the power supply and the required start-up time.

[0064] In FIG. 7, each disk drive is represented by a circle, and onedisk drive in each parity group is designated as a master and subjectedto rotation synchronization. Although the master can be any disk drivein the parity group, it has to be brought up to the rated rotationalspeed ahead of the other disk drives in the group. The method used whenthe nvention is applied in such a case will now be explained. There are4 parity groups (each parity group is in a single horizontal line) and 4power groups (each power group is in a vertical line). The disk drivesare or the same type as those in the first embodiment. In each paritygroup, the disk drive designated as the master is preferably started upbefore the others. In FIG. 7, the disk drives 411, 412, 413 and 414lying an the diagonal at the intersections between the respective paritygroups and power groups are selected as the masters. All of the mastersare simultaneously started up. Following this, the remaining disk drivesof group 420 are started up and thereafter the remaining disk drives ofgroup 430 indicated in the same figure (remaining means other than thosedisk drives already started up) are started up. The time variation inthe power supply currents of the respective power groups in this case isshown in FIG. 8, from which it will be noted that a maximum current of10 A suffices and the steady state current after start-up is SA. Ifstart-up should be carried out without application of the invention, 16Awould be necessary. The invention thus produces a pronounced effect inreducing power supply requirements.

[0065] In contrast to the above examples, FIGS. 9 and 10 respectivelyrelate to cases in which the number of parity groups is smaller andlarger than the number of power groups. When a master disk drive 440 forrotational svnchronlzation is designated in each parity group, thearrangement of FIG. 9 results in some power groups including no masterdisk drive 440 and that of FIG. 10 results in some power groupsincluding more than one master disk drive 440. When the invention isapplied to these arrangements, it suffices to establish the start-upgroups 450, 460 and 470 shown in these figures and after star up of themasters, to startup these groups 450, 460 and 470 in succession at atime interval equal to the time required for the disk drives to reachthe prescribed rotational speed following start-up. It can be easilyunderstood that the effect of the invention is obtainable with thisarrangement. Therefore, the start-up sequence is: the master disk drives440 are all first started up during a first period of time; and this isfollowed by a second period of time wherein the disk drives other thanthe master disk drives 440 are started up within start up group 450;thereafter, the disk drives other than the master disk drives 440 arethen started up in start up group 460; and thereafter the disk drivesother than the master disk drives 440 are started up in group 470.

[0066] In FIG. 11, magnetic disk drives 551-560 are represented ascircles. Since this example is aimed at ach eving a very high transferrate, a number of parity groups each constitluted of a plurality of diskdrives are arranged in parallel and reading and writing operations areconducted with respect to all of the disk drives simultaneously. Morespecifically, when data to be stored is received from the host 210, itpasses through an interface 230 to a first sequencer 510 where it issubdivided into units of an arbitrary size. These data units aretemporarily stored in first buffers 511-513 and then are furthersubdivided in sequencers 521-523. The subdivided data are written to themagnetic disks 551-560 via second buffers 531-540 and logical controlcircuits 541-550. This process is reversed during daia regeneration.

[0067] Although it is possible to provide each disk drive in thisarrangement with its own separate power supply, the number of powersupplies required would be very large. A better arrangement can berealized by taking advantage of the fact that each parity group(constituted by the disk drives under one of the sequencers II) includesa redundant or parity disk drive. If power supplies are provided so thateach supplies power to onlv one disk drive in each parity group,specifically if a power supply 571 is provided to supply power to diskdrives 551, 555, and 558, a power supply 572 is provided to supply powerto disk drives 552, 556 and 559 and so on, reading and wr ing will bepossible with ECC even if one of the power supplies should break down.It is thus possible to realize a system with high reliability.

[0068] Being common to the whole system, the interface 230, thesequencer 510, the buffers 511-513 and the sequencers 521-523, forensuring high reliability, preferably should be provided with a powersupply separate from that for the magnetic disk drives to establish adual power system. Even though the buffers 531-540 and control circuits541-550 are part of the same power groups, they are logical circuits andpreferably should have a different voltage power supply. It is, however,contemplated to supply them with stepped-down voltage from a powersupply for driving the disk drives.

[0069] The power groups constituted such that each power supply suppliespower to one magnetic disk drive in each part y group in the manner ofFIG. 11 are started wioth exactly the same method and arrangement as inFIG. 1 and the effect of the invention is thus manifested.

[0070] The power supplies of disk drives are sometimes equipped withbatteries for supplying power during power outages and emergencies. Whenthe invention is applied to such a battery backed up system, it reducesthe load on the batteries when they are used for starting up the disksystem and further upgrades system reliability.

[0071] In the foregoing, the invention was explained with resmect tosystems employing magnetic disk drives. It is obvious, however that theinvention can also be applied with good errect to systems employingoptical disk drives, hard or floppy disk drives, or the like insofar asthe spindle motors of the drives exhibit characteristics like thoseshown in FIG. 4.

[0072] Moreover, the description up to this point has been limited to apower-on sequence or start-up for the disk drive groups which enablespower to be supplied to the disk drives with high efficiency atstart-up. However, a disk drive puts an increased load on its powersupply from certain elements not only power-on but also 1) during seekoperation when a disk actuator equipped with a plurality of magneticheads operates to position the heads at target track positions on thedisks, and 2) when a read-write amplifier is operated for conductingread and write operations. Those operations also involve a risk of cowersupply overload should thel be conducted with rescect to al the dskdrives of a power group at the same time. When the present invention isapplied, however, since the disk drives are organized in groups and theoperational timing is shifted between the respective groups, overloadingof the power supplies can be avoided.

[0073] In accordance with this invention, since disk dr fir groups arestarted up one at a time, the power supplies for powering the diskdrives need not be larger than necessary for supplying current requiredby all the disk drives in steady state operation. Moreover, the timerequired for all of the disk drives to reach their rated rotationalspeed can be shortened.

[0074] Moreover, in a system which must as a whole be opera ted bybattery, the application of the present invention, through its effect ofreducing the tIme required for start-up and its effect of suppressingthe maximum load current, enables the use c small capacity batteries.

[0075]FIG. 12 (a) is a block diagram of a part of the system relating todata storage and FIGS. 12(b)- and 12(c) are views for explaining thedata storage. As shown in FIG. 12(a), a CPU 1 is connected by a bus toan arrav disk controller (ADC) 2 and an array disk unit (ADU) 3.

[0076] The ADU 3 has a plurality (six being specifically shown) ofparity groups 4, each of which has a group controller (GC) 5, four datadisk drives D (7) and one parity disk drive P (3). The system inputs andoutputs data between the data and parity disk drives 7, 8 and the CPU 1via data lInes 6. Electric current is supplied from a power supply 10 tothe resspective parity groups 4 via power lines 9. The number of datadisk drives 7 and parity disk drives 8 is determined in light of theamount of power the system is capable of supplying.

[0077] Each parity group 4 has a unit for generation of parity bits. Oneparity bit is generated from the data bits of the respective data diskdrives 7. Each of the data disk drives 7 and the parity disk drives 8 inthe parity groups 4 is of the structure illustrated in FIG. 13.

[0078] The disks of this drive rotate at 3,600 rpm (reauiring 16.5 msper revolution) and the data transfer rate from the disks is 3 MB/s.Data is recorded on concentric tracks on the disks. The track positionsare defined on each disk by fixed pcositions of a single R/W head 13.R/W heads #o to #8 are positioned at corresponding track positions onthe disk 12 by an actuator 11. The actuator 11 moves all of the R/Wheads 13 simultaneously and by the same distance. A single positioningoperation by the actuator 11 determines 9 tracks corresponding to theR/W heads 4,0 to #8 and these 9 tracks are collectively referred to as acylinder. The amount of data that can be read from one disk during onerevolution is called the track data capacity. Where this capacity is 35KB and there are 9 R/W heads per cylinder, the cylinder data capacitybecomes 35×9=313 KB.

[0079] Presuining a system of the type shown in FIG. 12(a), which hasfive parity groups 4, an examle will now be explained regarding a casein which the CPU 1 issues a 10 MB write reauest and the system conductsparallel writing of this data. (In this and the following embodiments,data read can be considered to be conducted in the same manner as datawrite). It will be understood from FIG. 14 that under the conditionsjust defined 2000 KB of data will be written to each of the five paritygroups 4 to handle the 10 MB request. Thus it is necessary to write 500KB of data to each of the four data disk drives 7 in each parity group4.

[0080]FIG. 15 is a time chart relating to the processing conducted withrespect to the data disk drives 7 and the parity disk drlves 8 of theparity groups 4 in this case.

[0081] Of the data transferred from the CPU 1 to a disk drive, data upto 315 KB is stored in cylinder #1 and the remaining 183 KB is stored incylinder #2. The head selector 14 first selects R/W head #0 according toa first seek operation and this head writes data to the correspondingtrack starting from the position of the index. This index serves asreference for the start of data writing. When writing of an amount ofdata correspondina to one revolution of the track under R/W head #o hasbeen compleed, R/W head #1 is selected and an amount of datacorresponding to one track is similarly written starting from the index.The switching between heads is done electrically and the time requiredtherefor is substantially negligible. The aforesaid write processing iscontinued with R/W heads 42, #2, #3, #4 . . . #8, after which theactuator 11 moves the R/W heads #0 to #3 in unison to the next cvlinderaccording to a second seek oceration. From the foregoing it will beunderstood that the time required for writina data to one cylinder is 9times the time for one revolution of the disks 12, i.e. about 150 ms.When the writing of data is continued to an adjacent cylinder in theaforesaid manner, it is only necessary for the actuator to carry out aseek aperation for moving the group of R/W heads to theadjacentcylinder. This requires a seek time of about 3 ms, during whichthe disks rotate by 3/16.6 revolution or approximately 1/5 revolution.There is thus a wait until the index next arrives and writing can begin.Including this wait time, therefore, there occurs a period of 16.6 ms(equal to the period of one disk revolution) during which data transferis impossible. In other words, it is necessary to complete the seekoceration within the period of one revolution. The amount of currentconsumed at this time is generally about 1.7 A per data disk drive 7 orparity disk drive 8. When a seek operation is conducted, however, theamount of current required for this operation is the total amount ofcurrent required up to a maximum of 4.5 A. Since each parity group 4includes four data disk drives 7 and one parity disk drive 8, i.e., atotal of 5 disk drives, which perform the same operations, the maximumamount of current required by the parity group 4 during seek operationbecomes 4.5A ×5=22.5A. The power supply 10 supplies this current to theGCs 5 of the respective parity groups 4 via the power lines 9.

[0082] As shown in FIG. 16, each of the CGs 5 comprises a commandprocessor 18, a disk control 19, a data processor 20 and a paritygenerator 21. The command processor 18 processes commands between theADC 2 and the group 4. Based on instructions received from the commandprocessor 18, the disk control 19 carries out specific control withinthe parity group 4. The data processor 20 handles the subdivision andrebuilding of data between the host 1 and the parity group 4. Associatedwith the data processor 20 is a parity generator 21 for generatingparity bits during data write and for reconstructing the data stored ina disk drive of the parity group 4 which has become unreadable becauseof a breakdown.

[0083] The disk control 19 synchronizes the rotation of the data diskdrives 7 and parity disk drive 8 of the parity group 4 and controls thetiming of the parallel processing within the cariy group 4 exactly as ifit -were being conducted with resoect to a single disk drive. It alsomaintains a check on whether the disk drives within the parity group 4are operating normally/ and manages the supply of power within theparity group 4.

[0084] Timing control for rotation synchronization among the paritygroups 4 is conducted by the ADC 2.

[0085] The ADC 2 subdivides the data transferred from the CPU 1 andallocates the subdivided data to the parity groups 4. It also controlsthe rotation synchronization timing of the respective parity groups 4for ensuring that it does not become necessary to conduct a seekoperation in any two of the parity groups 4 simultaneously. Thespecifics of this control are illustr ated in FIGS. 12(b) and 12(c).FIG. 12(b) shows the relationship among the indices 32 on the disksamong the rotation-synchronized data and parity disk drives 7, 8 of therespective parity groups 4. During parallel Processing the positions ofthe indices from which data write is started are deliberately offsetamong the different groups as shown in FIG. 12 (b).

[0086] The data transferred from the CPU 1 is subdivided bv the ADC 2and the subdivided data is simultaneously transferred to all of theparity groups 4. Within the respective parity groups 4, the datareceived is once stored in a buffer within the data processor 20 and isthen independently stored in the data and pari-ty disk drives 7, 8 ofthe respective groups. The data storage timing among the parity groups 4at this time is indicated in FIG. 12(c). The data storage time isindicated for five different parity groups, namely parity group #1through #5. With resecto each group, there is a data transfer time 40, aseek time 41, and a second data transfer time 42. These times are shownin synchronism with respect to each other and in synchronism with theseek current 43 according to the present invention when the indices areoffset, which seek current has a maximum value of 14 A, calculated bymultiplying the 2.8 A current required for each group, as explainedabove, times the five groups used in the example. In dotted lines, thereis shown the total seek current 44 that would be reauired if the indiceswere not offset, according to the prior art, which would require a seekcurrant cf 70 A calculated by multiplying the same 2.8 A for one diskdrive times the five disk drives that are in use at the same time timesthe five groups. Therefore, the advantage of offsetting the indices isclearly shown.

[0087] As shown in FIG. 15, when the amount of data to be stored to adata disk drive 7 in the parity groups #4 exceeds she capacity of onecylinder, it becomes necessary to conduct a seek operation for switchingto another cylinder and, as a result, there occurs a one-revolution waitperiod in the course of data storage during which processing cannot beconducted. Thus, as shown in FIG. 14, the rotation of the disk driveswithIn the respective parity groups 4 is synchronized such that theindex of each parity grouo 4 making up the array of parity groups 4 isoffset relative to the indices of the other parity groups 4 by at leastthe seek time. As a result, the timing of the scart of seek operationcomes to be offset among the parity groups 4. The amount of current thatwould have to be supplied to the system should the seek operation startiming not be o fset would amount to 22.5 A×5=122.5 A, the seek currentportion of which is 2.8 A×5×5=70 A. In contrast, applIcation of theinvention reduces this to 4.5 A×5=22.5 A, the seek current portion ofwhich is 2.8 A×5 =14 A. The method in which this offset of the seekstart timing is implemented is decided in light of the amount of currentthe system is capable of supplying. For example, where a current of 45 Ais available for supply to five parity groups 4, it is possible tocontrol the rotation synchronization timing of the parity groups 4 asshown in FIG. 17.

[0088] Since the invention enables operation at a lower seek current,the required current capacities of the power supply equipment and theemergency backup battery are also smaller, according to the presentinvention, which are considerable advantages.

[0089] The array disk system o0 FIG. 18 is like that of FIGS. 12, 11 butinstead of offsetting the indices among the groups, the embodiment ofFIG. 13 rotation synchronizes the disk drives of the parity groups 4,making up the array of parity groups 4 conducting parallel processing,such that the indices thereof are in phase. While this makes it possibleto carry out data storage simultaneously with respect to all paritygroups 4 constituting the array of parity groups 4 in FIG. 18, withoutthe present invention it causes all of the seek operations to occur atone time. In this embodiment, therefore, the seek operation start timingsignals are offset among the five parity groups 4, #1, #2, #3, #4, & #5in FIG. 18, making up the array of parity groups, such that the seekoperations start at different timings from the indices for thecorresponding different groups.

[0090] A seek operation ordinarily begins as soon as data storage to thecylinder has been completed up to the track under R/W head #8 In FIG.18, however, the seek operation start timing is deliberately offset bydifferent amounts among the differenr parity groups 4, respectively. Themethod in which the seek operation start timing is offset among theparity groups 4 will now be explained.

[0091] As shown in FIG. 15, when an amount of data exceeding thecapacity of a single cylinder is to be stored in a data disk drive 7 ofa parity group 4, a seek operation becomes necessary for moving to theadjacent cylinder.

[0092] The GC 5 begins storing data to the track concerned afterdetecting the index thereon. When a seek operation occurs in the courseof storing data, it takes about 3 ms to move the R/H heads to theneighboring tracks at which data storage is to be continued. By the timethe seek operation has been completed, the disk has rotated by 3/16.5revolution. When this period is up, the index has already passed bv thehead so that i- is necessary to wait until the index comes around again.

[0093] Thus, even though the seek operation itself is completed in 3 ms,in actuality a wait period (a period during which processing cannot beconducted) equal in length to the time recuired for one revolutIon ofthe disk occurs in the course or data storage. Since the system simplywaits during this period and cannot carry out any processing, this timecan be used for sequentially completing the seek operations in therespective parity groups 4.

[0094] In each parity group 4, the seek operations are managed by thedisk control portion 19 in the GC 5.

[0095] The ADC 2 sets the seek offset times for the respective paritygroups 4 making up the arrav of parity groups 4 in advance and forwardsinstructions indIcating these times to the disk control portions 19 ofthe GCs 5. Based on the instruction it receives, the disk controlportion 19 does not initiate a seek operation as soon as storage to thetrack under R/W head #8 has been completed but instead delays the startof the seek operation by the offset time indicated in the instructionfrom the ADC 2. Taking the specific example of conducting parallelprocessing with the five parity groups 4 shown in FIG. 18, while a seekoperation is started immediately after completion of storage to thetrack under R/W head 48 in the disk drives of group #1, the initiationof the seek operation in group #2 is delayed until the seek operation ingroup #1 has been completed. The seek operations in the remaining paritygroups 4 are similarly offset such that seek in group #3 is startedafter completion of seek in group 42, seek in group #4 after completionof seek in group #3, seek in group #5 after compiletion of seek in group#4, and so on. The data and parity disk drives 7, 8 of the respectiveparity groups 4 are rotation synchronized ad seek operation occurssimultaneously in all the data and parity disk drives 7, 8 of the sameparity group 4. The time required for the seek operation to move the R/Wheads to the adjacent track is about 3 ms. Thus where the seek starttimes are offset in the foregoing manner, the seek operations in all ofthe parity groups 4 involved in the parallel processing are completedwithin the one-revolution wait period during which data transferprocessing is impossie.

[0096] The seek operation tliming is automatically controlled bycontrolling the seek opera-4on of the 7Cs . In this mehod, even when itbecomes necessary to writ to the next cylinder because the amount ofdata exceeds the capacity or a single cylinder, the CPU 1 is not madeaware of this fact and the seek operation for moving to the adjacenttrack for continuing the writing of data is conducted automatically bythe GC 5.

[0097] The offsetting of the seek operations can be realized by thesimple expedient of using software techniques to offset the times atwhich the ADC 2 issues its commands. Moreover, since the disk indicesare all in alignment, control for rotation synchronization is easy toconduct.

[0098] Alternatively, it is possible to offset the locations at whichinformation indicating seek operation start time are recorded on thedisks.

[0099] With the operating systems (OS's) currently used in mainframecomputers, it is not possible with a single input/output request toprocess a large amount of data bridging a plurality of cylinders as isthe case in seek Embodiments 1 and 2. The maximum amount of data thatcan be processed by a single input/output request is limited to thecapacity of a single cylinder Thus where a large amount of data bridginga pluraliy of cylinders is processed, i- is necessary to allot oneinput/output request per cylinder and the host is required to issue asmany input/output requests as there are cylinders involved. When, forexample, parallel processing is conducted in the manner shown in FIG.13, namely where data is stored bridging cylinders 41 and #2 in fiveparity groups 4, the CPU 1 of the mainframe issues a single input/outputrequest for storage of data to cylinder #1 and then, after seekoperation has been completed following issuance of a seek command,issues another single input/output command for storage of data tocylinder #2.

[0100] In each instance, the CPU 1 issues only one seek command for allof the parity groups 4 making up the array conducting parallelprocessing. The ADC 2 issues this seek command to the GCs 5 of therespective parity groups 4 conducting the parallel processing but, asshown in FIG. 13(b), in doing so it offsets the issuance of the seekcommand among the GCs 5 of the different parity groups 4, thusoffsetting the seek operation start timing.

[0101] When this method is used, it becomes possible, similarly to thecase of the other embodiments, to complete seek operation with rescectto all of the parity groups 4 involved in the parallel processing withinthe one-revolution wait period during which data transfer processing isimpossible.

[0102] Data is stored at the same head address and the same cylinderaddress in all cf the parity groups 4 making up the array of paritygroups 4 conducting parallel processing. As shown in FIG. 19, in thepresent embodiment the head address from which data storage is startedis varied among the groucs at the time of starting data storage. Whilethe ADC 2 sets the same data storage start address for all parity groups4 making up the array of parity groups 4 in advance and forwards aninstruction indicating this time to the disk control portions 19 of theGCs 5, the command processing portions 18 of the GkS s changes this headaddress.

[0103] Consider, for example, the arrangement shown in FIG. 19(a) inwhich each of the data disk drives 7 and parity disk drives 8 of thefive parity groups 4 has five disks and read/write is conducted withrespect to only the upper surface of each disk. In this case, eachcylinder consists of five tracks. Where data is an amount equal to thecapacity of six tracks is to be stored in each group, in group #1writing of data is started from head #1, proceeds through heads #2, #3,4 and #5, and then, following a seek operation, continues at head #1 ofthe next cylinder. In group #2, storage starts from head #2, proceedsthrough heads #3, #,4r4 and #5, and then, after a seek operation,continues at heads #1 and #2 of the next cylinder. In group #3, storagestarts from head #3, proceeds through heads #4 and #5, and then,following a seek operation, continues at heads #1, #2 and #3 of the nextcylinder. In group #4, storage starts from head #4, proceeds throughhead #5, and then, following a seek operation, continues at heads 41,#2, #3 and #4 of the next cylinder. In group #5, storage starts fromhead is and then, following a seek operation, continues at heads #1, 42,#3, #4 and #5, of the next cvlinder. Varying the head address for thestart of data storage amona the different parity groups 4 in this manneralso causes the seek operation start timing to be offset among thedifferent parity groups.

[0104] The ADC 2 sends the same data storage start address instructionto the disk control pcri ons 19 of the GCs 5 of all the parity groups 4making up the arrav and the individual command processing portions 18change this address. It is obvious, however, that the same effect canalso be obtained by having the ADC 2 change the data storage startaddress and send instructions based on the changed address to the GIs5-.

[0105] In the above description, it was explained how, by offsetting theseek operation start timing among different parity groups 4, theinvention achieves its object of preventing the occurrence of the largecurrent which arises when a large number of seek operations occursimultaneously in an array of parit-y groups 4 conducting parallelprocessing. This same thinking can obviously also be applied foroffsetting read and write start timing so as to reduce the amount ofcurrent required during these operations.

[0106] The head addresses can be varied by software techniques, which isadvantageous in that it expedites the control for seek operation offset.

[0107] While the foregoing relates to cases in which the seek starttiming is offset among the groups, the same results can be obtained byoffsetting the seek operation start timing among the disk drives in oneand the same group.

[0108] Application of the invention reduces the current to be suppliedto the array disk system and also reduces the capacity required of abattery provided for preventing loss of data owing to power outages andthe like. It thus becomes possible to provide battery backup over a longperiod of time with a battery of relatively small capacity, thusenhancing reliability aganist the risk of data loss resulting from poweroutages. Furthermore, the invention also makes it possible to reduce thesize of the system's power supply equipment.

[0109] With respect to FIG. 19(b), each of the first and second datatransfer periods 40, 42 respectively for the groups #l to #5, is dividedby vertical dashed lines. For group #1, data transfer period 40 isdivided into subperiods for head #1, head #2, head X3, head #4, head #5respectively proceeding from left to right for a first cylinder and thesecond data transfer period 42 is for head #1 of cylinder 2. Withrespect to the second parity group #2, the data transfer period 40 isdivided into 4 subperiods respectively for head #1, head 42, head #3,head #4 of cylinder #1, and the second data transfer period 42 isdivided into two subperiods for head #1 and head #2 of cylinder 42. Withrespect to parity group #3, #4, #5, the data transfer periods 40 arerespectively divided into 3, 2 and 1 subceriods for head numbers 1, 2, 3head numbers 1, 2 and head numbers 1, respectively, each for cylinder 1;the second data transfer period 42 is divided respectively into 3, 4 and5 subperiods respectively for head numbers 1, 2, 3 head numbers 1, 2, 3,4head numbers 1, 2, 3, 4, 5 each for cylinder #2 of the respectiveparity groups. The seek current 41 for each group is shown. W4th this,it is seen that the current 43 has five current surges at timings 41, asshown, each of which is 14 amps, obtained by multiplying the 2.8 amps by5. It is noted that according to the present invention the currentsurges do not overlap and therefore do not reinforce each other.

[0110] While a preferred embodiment has been set forth along withmodifications and variations to show specific advantageous details ofthe present invention, further embodiments, modifications and variationsare contemplated within the broader aspects of the present invention,all as set forth by the spirit and scope of the following claims.

1. An array disk system for use with a host deviie, comprising: powersource means providing current to said disk drives; a plurality of diskdrives in an array and each disk drive having at least one element thatdraws steady state current during normal operation and larger transientcurrent during transitional operation from said power source means; saidelements including a plurality of disks mounted for steady staterotation about respective axes durina normal operation and movable fromrest to steady state operaj on rotation during transitional operation,and a plurality of heads being substantially stationary to the axis ofrotations on the respective disks during steady state operation andmovable radially relative to the axis of rotation of the correspondingdisk during transitional operation; means responsive to a signalcommanding initiation of transient operation for a pluralitv of likeones said el_ements for offsetting the initiation of at least twotransitional operations sufficiently so that their transitionaloperations do not overlap to a substantial extent and so that thetransient current demands on said power source means are reduced overtile transient current demands that would exist without the offsetting.2. The array disk system according to claim 1, wherein identical ones ofsaid elements are arranged in groups, with each group containing apluralitv of said elements; and said means for offsetting offsets theinitiation of the transitional operation between two groups.
 3. Thesystem of claim 2, further comprising: means subdividing serial data tobe written into a plurality of units of subdivided data; and means forreading and writing the units of subdivided data in parallel withrespect to the pluraliy of disk drives.
 4. The system of claim 3,wherein said means for subdividing includes a sequencer means fordividing the data into a first plurality of subdivided data, a firstpluralit4w of buffers respectively receiving the first plurality ofsubdivided data, a plurality of second sequencers respectively furthersubdividing the subdivided data from said buffers, and a furt-herplurality of buffers receiving the further subdivided data respectivelyand communicating with respective disk drives in parallel.
 5. The svstemof claim 4, wherein said disk drives are divided into a plurality ofparity groups, with each parity group including a plurality of said diskdrives as data disk drives and a single disk drive as a parity diskdrive; and means for synchronizing the rotational speed of all the diskdrives within a parity group with reference to one of the disk driveswithin the parity group acting as a master disk.
 6. A disk system foruse with a host device, comprising: a plurality of disk drives and eachhaving a read head and a plurality of tracks; controller means forcontrolling disk rotation, read head position and data processing, forconducting reading cf data with respect to the host device and theindividual disk drives; said controller means varying the start c; seekoperations for moving the read heads to change the track positions atwhich the read heads are located so that peak seek operation currentrequirements do not substantially overlap to reduce maximum currentrecuirements of the disk system.
 7. The disk system according to elaim6, including indices provided on the disk drives as references for thestart of data read and wherein said controller means includes means forrotation-synchronizing the disk drives such that position of the indicesare offset among the disk drives.
 8. The disk svstem according to claim6, wherein said controller means deliberately offsets the seek operationstar timing among the disk drives and said disks drives have alignedindices.
 9. The array disk system according to claim 6, wherein saidcontroller means varies timing of head addresses for the stark of dataread among the disks and said disk drives have aligned indices.
 10. Anarray disk system for use with a host device, comprising: a pluralityofL disk drives in an array and each having read/write heads; controllermeans for controlling disk rotation read/write head position and dataprocessing, for conducting parallel read/write of data bycombining/subdividing data transferred with respect to the host device,and for simultaneously exchanging the subdivided data in parallel wifthrespect to the individual disk drives; said disk drives being subdividedinto a plurality of parity groups; and said controller means varying thestart of seek operations for moving the read/write heads to change thetrack positions at whIch the read/write heads are located among at leastsome of the parity groups so that peak seek operation currentrequirements of the parity groups do not substantiallv overlap to reducemaximum current requIrements of the array disk system.
 11. The arraydisk system according to claim 10, wherein said controller meansincludes means for rotation-synchronizing the disk drives such thatposition of indices provided on the disk drives as references orn thestart of data read/write are offset among the parity groups.
 12. Thearray disk system according to claim 11, wherein said controller meansincludes buffer means which store subdivided data simultaneouslytransferred to the respective parity groups and conducts offsetread/write processing of the data relative to the disk drives inaccordance with the positional offset of the indices.
 13. The array disksystem according to claim 10, wherein said controller means deliberatelyoffsets the seek operation start timing among the groups and said diskdrives have aligned indices.
 14. The array disk system according toclaim 10, wherein said controller means further varying head addressesfor the start of data read/write among the groups and said disk driveshave aligned indices on the disks among the groups.
 15. The array disksystem according to claim 10, wherein said controller means controlssuch that within each group the timing of the start of the seekoperations is the same for all of the disk drives.
 16. The array disksystem according to claim 10, wherein said controller means controlssuch that within each group the timing of the start of the seekoperations is offset among at least some of the disk drives.
 17. Thearray disk system according to claim 16, wherein said disk drives haveindices, as references for the start of data read/write, and saidcontroller means includes means for rotation-synchronizing the diskdrives with respect. to the position of the indices.
 18. The array disksvstem according to claim 10, wherein said controller means includesbuffers storing subdivided data simultaneously transferred to therespective groups and conducts read/write processing of the data betweenthe buffers and the groups in accordance with the positional offset ofthe indices.
 19. The array disk system according to claim 70, wnere4nsaid disk drives have indices as references for the start of dataread/write, and said controller means deliberately offset the seekoperation start timing among the disk drives of a group, withoutoffsetting the positions of the indices on the disks.
 20. The array disksystem according to claim 10, wherein said disk drives have indices asreferences for the start of data read/write, and said controller meansoffsets the head addresses for the start of data read/write among thegroups W4- hout offsetting the positions of the indices on the disksamong the disk drives of a group.
 21. In a method of controlling anarray disk system to processes data in parallel among a plurality ofdisk drives; controlling the arrav disk system so that seek operationsfor moving transducer heads to change track poser ons at which thetransducer heads are positioned are prevented from occurringsimultaneously in at least some of the disk drives so that peak currentrequirements for seek of the at least some of the disk drivessubstantially do not overlap.
 22. The method of controlling an arraydisk system according to claim 21, wherein said controlling includesoffsetting the position of indices on the disks.
 23. The method ofcontrolling an array disk system according to claim 22 includingdividing the disk drives into groups, each consisting of a plurality ofthe disk drives; and said controlling is such that seek operation isprevented from occurring simultaneously among the groups, and seekoperations commanded to simultaneously occur are controlled to occur indifferent groups at different times within the period of one diskrevolution and all of the seek operations occurring at different timesare completed within the same period oL one disk revolution.
 24. Themethod of controlling an array disk system according to claim 21,wherein said controlling includes offsetting the seek oceration starttiming among disk drives.
 25. The method of controlling an array disksystem according to claim 21, wherein said controlling includesoffsetting head addresses for read/write starting among disk drives. 26.The method of controlling an array disk system according to claim 21,including dividing the large number of disk drives into groups, eachgroup having a plurality of disk drives; wherein said controllingprevents seek aperation from occurring simultaneously among the groups;and said controlling and dividing being such that seek operations startin different groups at different times within the period of a singledisk revol uton for all of the groups and all of the seek operationsstarting at different times are completed within the period of a singledisk revolution.
 27. A method of operating a disk system having a piuraltv of disk drives, comprising the steps of: subdividing serial data tobe written inno a plurality of units of subdivided data; reading andwriting the units of subdivided data in parallel with respect to theplurality of disk drives; driving the plurality of disk drives with aoower source providing a steady state current during steady statedriving of the disk drives; dividing the disk drives into a pluralily ofstart-up groups so that at least a first one of the start-up groups hasa plurality of the disk drives; at the time of power-on, starting up thefirst one of the start-up groups with current from the power source fora first period of time until the first start-up group attains a steadystate condition; at about the end of the first period of time, startingup a second one of the start-up groups with current from the powersource for a second period of time until the second start-up groupattains a steady state condition while continuing to rotate the firstone of the start-up groups al steady state; repeating the preceding stepsequentially for the remaining, if any, start-up groups until all of thedisk drives have attained steady state driving.
 28. The method of claim27, wherein said step of dividing provides the number of disk drives inthe first start-uo group to be equal to the total number of disk drivesdivided bv the ratio of the initial current required by a single diskdrive during start-up to the steady state current required by a singledisk drive.
 29. The method of claim 27, including: designating aplurality of the disk drives as master disk drives; during steady state,synchronizing rotation of the disk drives other than the master diskdrives with the master disk drives; and wherein said step of dividingforms the first one of the start-up groups to comprise at least all ofthe master disk drives.
 30. The method of claim 27, includingmagnetically writing and reading data to and from magnetic disksrespectively driven by the disk drives.
 31. The method of claim 27,including optically reading data from optical disks respectively drivenby the disk drives.
 32. The method of claim 27, wherein said step ofdividing is conducted such that the second one of the start-up group hasless disk drives than the first one of the startu groups.
 33. The methodof claim 32, wherein said step dividing is conducted such that a thirdone of the start-up groups of disk drives has a less number of diskdrives than the second one of the start-up groups of disk drives. 34.The method of claim 27, wherein said step of dividing is conducted sothat each subsequent start-up group has a number of disk drives that isnot greater than the number of disk drives in any preceding start-upgroup.
 35. The method of claim 27, wherein said step of dividingdetermines the number of disk drives in each start-up group to be equalto (the current capacity of the power supply minus the steady statecurrent of a single disk drive times the number cdisk drives in apreceding start-up group, which have reached steady states) divided by (the start-up current required by a single disk drive), rounded down to awhole integer.
 36. The method of claim 35, wherein said step of dividingforms each start-up group with the maximum number of disk drives thatcan be started up in its time period so that the start-up current forthe start-up group plus the steady state currents of the disk drives ofpreceding start-up groups does not exceed the current capacity of thepower supply.
 37. The method of claim 36, performed with respect to eachof a plurality oa power supplies and separate disk drives respectivelyconnected to the power supplies in an array of disk drives and powersupplies.
 38. The method of claim 27, wherein said step of dividingforms each start-up group with the maximum number of disk drives thatcan be started up in its time period so that the start-u current for thestart-up grou plus the steady state currents of the disk drives ofpreceding start-up groups does not exceed the current capacity of thepower supply.
 39. The method of claim 27, wherein said step ofsubdividing the data passes the data through a sequencer to a pluralityoa first buffers, passes the data from each first buffer to a secondsequencer, and divides the data in each second sequencer to subdata; andpasses each subdata through a separate butfer and logical control to adisk drive in parallel for all the subdata from at least the secondsequencer.
 40. The method of claim 39, further including steps of:dividing the disk drives among a plurality of power groups for steadystate driving to form an array disk system, which power groups aredifferent from the start-up groups and so that each power group containsdisk drives provided with data. respectively from all of the secondsequencers, so that all the disk drives are divided among the powergroups without any one disk drive being in more than one power group andso that only one disk drive communicating subdata with a secondsequencer is contained within one power group; and wherein saidsubdividing, reading and writing, driving, dividing, starting up andrepeating are conducted with respect to each of a plurality of powersupplies and corresponding plurality of groups of the disk drives. 41.The method of claim 40, wherein said step of dividing provides the firststart up group with a number of disk drives not exceeding the currentcapacity of the power supply divided by the ratio of start up current tosteady state current of a single disk drive.
 42. The method of claim 41,including parity dividing the disk drives into a plurality of paritygroups so that the parity groups contain disk drives different from,respectively, the power groups.
 43. The system of claim 27, wherein saidstep of dividing provides the number of disk drives in each start upgroup to not exceed the reserve current divided by the scare up currentof a single disk drive, wherein the reserve current is equal to thecurrent capacity of the power supply minus the number of disk drives inprior start-up groups times the steady state current of a single diskdrive.
 44. The system of claim 43, including parity dividing the diskdrives into a plurality of parity groups so that the parity groupscontain disk drives different from, respectively, the start-up groups.45. The system of claim 44, wherein each parity group includes a masterdisk and wherein said step of dividing the disk drives provides thefirst start-up group to contain all of the master disk drives.
 46. Themethod of claim 44, including providing a plurality of power supplies,with each power supply supplying power to only a single disk drive ofeach and every parity group, and wherein said steps of subdividing,reading and writing, driving, dividing, starting up and repeating areconducted with respect to all the disk drives provided with power byeach power supply.
 47. The method of claim 27, wherein said step ofdividing is conducted so that each subsequent start-up group has anumber of disk drives that is not greater than the number of disk drivesin any preceding start-up group; said step of dividing determines thenumber of disk drives in each start-up group to be equal to (the currentcapacity of the power supply minus the steady state current of a singledisk drive times the number of disk drives in a preceding start-upgroup, which have reached steady states) divided by (the start-upcurrent required by a single disk drive) , rounded down to a wholeinteger; and said step of dividing forms each start-un group with themaximum number of disk drives that can be started up in its time periodso that the start-up current for the sr-artup group plus the steadystate currents of the disk drives of preceding start-uc groups does notexceed the current capacity o: the power supply.
 48. The method of claim47, wherein said step of subdividing the data passes the data throuch asequencer to a plurality of first buffers, passes the data from eachfirst buffer to a second sequencer, and divides the data in each secondsequencer to subdata; passing each subdata through a separate buffer andlogical control to a disk drive in parallel for all the subdata from atleast the second sequencer; dividing the disk drives among a pluralityof power groups for steady state driving, which power groups aredifferent from the start-up groups and so that each power group containsdisk drives provided with data respectively from all of the secondsequencers, so that all the disk drives are divided among the powergroups without any one disk drive being in more than one power group andso that only one disk drive communicating subdata with a secondsequencer is contained within one power group; said subdividing, readingand writing, driving, dividing, starting up and repeating are conductedwith respect to each of a plurality of power supplies and correspondingplurality of groups ofl the disk drives; and parity dividing the diskdrives into a plurality of parity groups so that the parity groupscontain disk drives different from, respectively, the power groups andthe start-up groups
 49. The method of claim 48, wherein each paritygroup includes a master disk; wherein said step of dividing the diskdrives provides the first start-up group to contain all of the masterdisk drives; providing a plurality of power supplies, with each powersupply supplying power to only a single disk drive of each and everyparity group; and wherein said steps of subdividing, reading andwriting, driving, dividing, starting up, and repeating are conductedwith respect to all the disk drives provided wizh cower by each powersupply.
 50. A disk system, comprising: a plurality of disk drives; meansfor subdividing serial data to be written into a plurality of units ofsubdivided data; means for reading and writing the units of subdivideddata in parallel with respect to the plurality of disk drives; means fordriving the plurality of disk drives with a power source providing asteady state current during steady state driving of the disk drives;means for dividing the disk drives into a plurality of start-up groupsso tha- at least a first one of the start-up groups has a plurality ofthe disk drivtes; means for, at the tIme of power-on, starting up thefirst one of the start-up groups with current from the power source fora first period of time until the first start-up group attains a steadystate condition; means for, at about the end of the first period oftime, starting up a second one of the start-up groups with current fromthe power source for a second period of time until the second start-upgroup attains a steady state condition while continuing to rotate thefirst one of the start-up groups at steady state; and means forrepeating the starting sequentially for the remaining, if any, start-upgroups until all of the disk drives have attained steady state drivIng.51. An array disk system for use with a host device, comprising: aplurality of disk drives in an array and each having read/write heads;controller means for controlling disk rotation, read/write head positionand data processing, for conducting parallel read/write of data bycombining/subdividing data transferred with respect to the host device,and for simultaneously exchanging the subdivided data in parallel withrespect to the individual disk drive; said disk drives being dividedinto a plural iy of parity groups; and each parity group including amaster disk; means responsive to a start up signal for starting up allof the master disk drives as a part of a first start up group, andthereafter starting up the remaining disk drives after the first groupof disk drives has substantially reached steady state condition; andmeans for controlling the rotational speed of the disk drives other thanthe master disk drives with reference to the master disk drives, foreach parity group.
 52. The array disk system according to claim 51,wherein said controller means includes buffers and is provided with dataprocessor means which store subdivided data simultaneously transferredto the respective groups in the buffers within the respective groups andconducts read/write processina of the data in accordance with an offsetsufficient to prevent overlap of seek operations of the heads.
 53. Thesystem of claim 52, wherein said controller means includes sequencers,and subdivides the data through a sequencer to a first plurality of thebuffers, passes the data from each first buffer to a second sequencer,divides the data in each second sequencer to subdata and passes eachsubdata through a separate second one of the buffers to a disk drive incarallel for all the subdata from at least the second sequencer.
 54. Thesystem of claim 53, including a plurality of power supplies; wherein thedisk drives are divided among a plurality of power groups for steadystate driving respectively by the power supplies, which power groups aredifferent trom the start-up groups and so that each power group containsdisk drives provided with data respectively from all of the secondsequencers, so that all the disk drives are divided among the powergroups without any one disk drive being in more than one power group andso that only one disk drive communicating subdata with a secondsequencer is contained within one power group.
 55. The device of claim54, wherein the first start up group for each power supply has a numberof disk drives not exceeding the current capacity of the power supplydivided by the ratio of start up current to steady state current of asingle disk drive.